Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/12

viii of miles of sea show remarkable resemblances that can only be reasonably explained if these sediments were laid down in close proximity to one another and under practically identical conditions.

The evidence afforded by the variation from point to point of the gravitational and magnetic forces leads us to the conclusion that the oceans and continents do not simply represent, as was formerly supposed, local and temporary depressions and elevations of the surface, but correspond to essential differences in the composition of the earth’s crust.

The rocks of the continental masses consist for the most part of the acid plutonic rocks, granite and its foliated form gneiss. The sedimentary, metamorphic, and basic igneous rocks, though they often play a conspicuous rôle at the surface are essentially subordinate in amount. The continental rocks as a whole are comparatively low in density and are composed predominantly of silica, alumina, and the alkalis, and are sometimes referred to collectively as sial (that is to say, silica plus alumina). [Note.—Suess employed the term “sal,” but most people will agree with Professor Wegener in adopting a suggestion of Professor Pfeffer that the form “sial,” which has not, like “sal,” an irritating resemblance to the latin for salt, should be substituted.]

There is good reason for believing that the rocks forming the substratum of the ocean bed are more basic in composition and contain a large proportion of magnesia, iron oxide, and lime, and that similar rocks or magmas of the same composition underlie the sial of the continents and probably form a zone of the earth’s substance some fifteen hundred kilometres in depth. They constitute the sima (silica plus magnesia), in contradistinction to the sial.

The thickness of the continental sial is estimated